This invention relates to a container for housing a nuclear reactor, and more particularly to an improved container capable of withstanding an earthquake and protecting the container against jet streams of steam and water and the percussive forces of bodies moving at high rates of speed under emergency conditions.
As diagrammatically shown in FIG. 1, a prior art container 1 for housing a nuclear reactor is contained in a biological shield wall 2 integral with the foundation mat 9 for supporting the building for the nuclear reactor. The biological shield wall and the foundation mat 9 are usually made of reinforced concrete. A pedestal 3 is mounted on the bottom of the container 1 for supporting the pressure vessel 4 of the reactor.
Various pipes including a main steam pipe 5 are connected to the pressure vessel 4, and means 6 for preventing the whipping of the main steam pipe 5 and other pipes, not shown, for example, pipes for circulating the water in the reactor, is supported by a thermal shield wall 7 which is mounted on the pedestal 3 so as to surround the pressure vessel 4. Since the means 6 for preventing the whipping of the main steam pipe 5 is supported by the pedestal 3 through means of the thermal shield wall 7, in a cantilever fashion, not only the means 6 but also the thermal shield wall 7 and the pedestal 3 are required to be highly rigid with the result that they become massive and decrease the free space in the container 1. In addition to the various pipes described above, various valves, electric motors, and instrumental devices are also contained in the container 1.
Should such pipes break for some reason, high pressure water and steam eject are ejected through the broken pipe like a jet stream and impinge upon the inner wall of the container 1. Furthermore, in an emergency, some of the various elements described above will be thrust against the container with considerable force and at high rates of speed.
In prior art container for housing a nuclear reactor, it has been necessary to fabricate the container 1 with steel plates having sufficient mechanical strength and thickness to withstand the large forces caused by the aforenoted jet streams or entrained component elements. But according to the present standards and or codes, it is required to perform post-assembly weld heat treatments for container 1 when the same is fabricated with plates having a thickness greater than a predetermined limit.
According to the present practice, in order to avoid the post-assembly weld heat treatments the container 1 has been encased within the biological shield 2 with a small gap 8 defined therebetween. Consequently when the container is subjected to large forces, the integrity of the container, fabricated with plates having a thickness within the standard and or permissible by the code, can be maintained.
Considering the resistance of the structure against earthquakes, the vibration of the foundation mat is transmitted directly to the pressure vessel 4 as well as to a plurality of fuel rods, not shown, contained therein and extending through the rigid container 1 and the pedestal 3. Accordingly, the fuel rods, having a natural vibration frequency relatively close to that of the reactor building, resonate so as to vibrate greatly. Consequently it has been difficult to prevent vibration of and damage the fuel rods.
During the fabrication of the structure, since the gap 8 between the reactor container 1 and the biological shield 2 is small, as above described, it has been impossible to approach the entire outer surface of the container 1. Accordingly, it has been impossible to perform a leak test once the container 1 has been installed. In other words, unless the leak test has been completed, the biological shield 2 can not be fabricated, thus prolonging the overall construction period of the reactor plant.
Moreover, as the means 6 for preventing the whipping of the pipe is supported on the pedestal 3 through means of the thermal shield wall 7, it has been necessary to construct the pedestal 3 so as to have sufficient strength.